Complain About a Bill That Doesn't Match What You Were Sold

I need to write a complaint to [provider name] about my latest bill. I was told I'd be paying [price you were quoted] per month for [service or package name], but my bill has come through at [amount on your bill]. I signed up on [date you signed up] and was sold this deal by [how you were sold it — e.g. over the phone, in store, via their website, by a door-to-door salesperson]. [Add any extra details, e.g. "I was told this price was fixed for 12 months" or "I have a confirmation email showing the agreed price"]. Please write me a firm but polite complaint email or letter that clearly states what I was promised, what I've actually been charged, and asks them to honour the original price. Include a request for a written response within a reasonable timeframe. Keep the tone confident and assertive but not aggressive. Around 300 words.